"The Little Book of Plagiarism" by Richard A. Posner; Pantheon; 116 small pages; $11

Many, if not most of the sentences we speak and write have never been produced before, and will never again see the light of day or dark of night. They're unique, even though they contain familiar, even shopworn phrases like "light of day" and "dark of night." (Exceptions: ordinary "housekeeping" sentences - "Your mother called," "That lane's moving faster," "When are the Chargers going to can Schottenheimer?")

Oddly, it is this, the very nature of language, that now makes it easier to detect word theft, as Posner points out in "The Little Book of Plagiarism."

While desperate, lazy or just plain old-fashioned dishonest students may be able to buy and download term papers from the Internet, Posner writes, professors now counter with programs that scan for even brief runs of identical words that appear in millions of online reports and documents. A match of more than a few words in a row hoists an electronic red flag.

Posner packs a lot into a few pages. Most interesting is his thumbnail history of the very concept of plagiarism. Shakespeare and his contemporaries, for example, borrowed (gentle word, that) plots, characters and even phrasing from well-known works, in part because audiences were comfortable with the familiar, and in part because creativity - or more specifically, an individual's contribution to a play or even to a painting - was viewed differently in those days; art was seen as more of a collaborative effort.

He also teases out the differences between plagiarism and copyright infringement. Sometimes they are one in the same, but not always; student papers are not published, for example, and authors may plagiarize from sources that are no longer protected by copyright. Still, others can in fact be harmed by such behavior: fellow students in the class, for example, if the professor grades on a curve.

Professorial plagiarism can damage colleagues, in that published papers can lead to professional leapfrogging.

In short - actually, the whole book is pretty much in short - "Plagiarism is a species of intellectual fraud. It consists of unauthorized copying that the copier claims ... is original with him and the claim causes the copier's audience to behave otherwise than it would if it knew the truth."

Plagiarism, while sometimes a matter of cut and paste, is not necessarily cut and dried; it can't always be defined by the now-standard I-know-it-when-I-see-it criterion Justice Potter Stewart applied to pornography.

Take the recent (too recent for Posner's book) brouhaha over British novelist Ian McEwan's mining of Lucilla Andrews' war memoirs for details he reproduced - with little alteration - in his acclaimed 2001 novel "Atonement." McEwan noted Andrews' work in the book's Acknowledgments, but does that extenuate?

Fellow novelists leaped to McEwan's defense. Even Thomas Pynchon peeked out to issue a (typewritten!) note explaining that, "Oddly enough, most of us who write historical fiction do feel some obligation to accuracy. ... Unless we were actually there, we must turn to people who were, or to letters, contemporary reporting, the encyclopedia, the Internet, until, with luck, at some point, we can begin to make a few things of our own up. To discover in the course of research some engaging detail we know can be put into a story where it will do some good can hardly be classed as a felonious act - it is simply what we do."

And yet McEwan, in one passage, trod closely enough in Andrews' prose footprints that I felt I knew it when I saw it. Not a felony, perhaps, but a certifiable, if not actionable, misdemeanor.

Wham! Good lord, what was that? Ever been blindsided? Something just comes out of nowhere and flattens you, and you never saw it coming?

That's what Posner does on Page 94, discussing the "rehabilitation" of historian Doris Kearns Goodwin: "I note that one reason for the ambivalence of reactions to plagiarism is that the Left, which dominates intellectual circles in the United States, is soft on plagiarism. Notions of genius, of individual creativity, and of authorial celebrity, which inform the condemnation of plagiarism, make the leftist uncomfortable because they seem to celebrate inequality and 'possessive individualism' (that is, capitalism)."

Picking yourself up off the ground, you look around for the yellow flag: Any decent ref would call him for clipping.